Fossil fuels, such as petroleum and coal, contain sulfur (hereinafter represented by “S”). Hence, when a fossil fuel is combusted, S is oxidized, and sulfur oxides such as SO2 are mixed in an exhaust gas. When the temperature of an exhaust gas decreases in a pipe, such as a gas duct, a chimney pipe, or an exhaust gas desulfurizer, fitted to an apparatus (such as an industrial boiler) in which a fossil fuel is combusted, this SOx gas reacts with moisture in the exhaust gas to form sulfuric acid and, as a result, dewdrops thereof are formed on an inner surface of the pipe. This sulfuric acid in the form of dewdrops enables corrosion (hereinafter referred to as “sulfate corrosion”) of the pipe to progress.
Various techniques to prevent the sulfate corrosion have been investigated and, for example, there has been used a technique in which a pipe for an exhaust gas is formed from low-alloy steel or a technique in which the temperature of an exhaust gas is controlled to 150° C. or more.
However, by the techniques described above, although the sulfate corrosion may be suppressed, it is difficult to stop the progression thereof.
In recent years, concomitant with an expansion of automobile market in Asia, iron steel has been increasingly in demand, and the amount of fossil fuels consumed in blast furnaces, heat treat furnaces, and the like of steel industry has also been increased. Hence, development of techniques to prevent the sulfate corrosion has become an urgent requirement in the steel industry. In addition, since gasoline contains S, the sulfate corrosion is also generated in pipes for exhaust gases emitted from automobile engines. Accordingly, exhaust gas pipes of automobiles also require a technique to prevent the sulfate corrosion. In addition, many of these pipes are subjected to a severe bending work.
Since high-temperature exhaust gases pass through exhaust gas pipes of blast furnaces, heat treat furnaces, and automobiles, low-alloy steel has not been used to prevent high-temperature oxidation, but ferritic stainless steel has been used in many cases. Hence, various techniques to improve the resistance against the sulfate corrosion (hereinafter referred to as “sulfate corrosion resistance”) of ferritic stainless steel have been studied.
For example, in Japanese Unexamined Patent Application Publication No. 56-146857, a technique has been disclosed in which acid resistance is improved by decreasing the S content of ferritic stainless steel to 0.005 mass percent or less. However, in Japanese Unexamined Patent Application Publication No. 56-146857, the acid resistance is investigated by dipping ferritic stainless steel in boiling hydrochloric acid, and the sulfate corrosion resistance has not been disclosed.
In Japanese Unexamined Patent Application Publication No. 7-188866, a technique has been disclosed in which, to suppress intergranular corrosion caused by nitric acid, the contents of C and N of ferritic stainless steel are decreased, and the contents of Mn, Ni, and B are also defined. However, according to the generation mechanism of intergranular corrosion caused by nitric acid, an environmental potential becomes positive due to the presence of nitric ions, and hence the breakage behavior of a passivation film of stainless steel and the stability of corrosion products are different from those caused by the sulfate corrosion. Accordingly, to apply the technique disclosed in Japanese Unexamined Patent Application Publication No. 7-188866 to prevent the sulfate corrosion, further study must be carried out.
To improve the formability of a ferritic stainless steel sheet, there has been investigated a technique in which the amounts of C and N are considerably decreased in a refining step of molten steel which is used as a raw material or a technique in which C and/or N is stabilized by the formation of carbides and/or nitrides by addition of Ti and/or Nb to molten steel. As a result, a ferritic stainless steel sheet having superior deep drawing characteristics to those of an austenite stainless steel sheet has been developed. However, according to a related ferritic stainless steel sheet having superior deep drawing characteristics, the formability by a deep drawing work, which is evaluated, for example, by a Lankford value (so-called r value), is improved.
In addition, to reduce the degree of rough surface (so-called “orange peel”) at a bent part formed by stretch forming, a technique has been investigated to improve a method for forming a ferritic stainless steel sheet into a predetermined shape (for example, see Japanese Unexamined Patent Application Publication No. 2005-139533). However, the rough surface at a bent part is not only generated by stretch forming but is also generated, for example, by a bending work, and research on a technique for reducing the degree of rough surface at a bent part by improving components of a ferritic stainless steel sheet and a manufacturing method therefor has not been sufficiently carried out.
The rough surface is a collective term including various surface defects, and in a ferritic stainless steel sheet, a rough surface, which is called “ridging,” is frequently generated. The ridging indicates a surface defect which is caused by the difference in deformation between individual textures which is generated when the textures are processed in a rolling direction generated by rolling. Although steel which suppresses the generation of ridging has been disclosed in many reports, even when the steel described above is used, a rough surface at a bent part may be apparently observed in some cases. Accordingly, it is believed that the generation mechanism of the rough surface at a bent part is different from that of the ridging, and hence measures suitable for the respective problems are separately required. In particular, when a bending work is performed at an angle of 90° or more, the rough surface is apparently generated.
Accordingly, it could be helpful to provide a ferritic stainless steel sheet and a method for manufacturing the same, the ferritic stainless steel sheet having a superior sulfate corrosion resistance even in a high-temperature atmosphere and further having a low degree of rough surface at a bent part formed by a bending work performed at an angle of 90° or more.